1. Field of the Invention
The present invention relates to a screw compressor for accommodating low pressure ratios and pressure variation and the operating method thereof in the case where the screw compressor is applied in a use for compressing gas of relatively high pressure to a constant discharge pressure or for compressing gas of which suction pressure varies from low to near discharge pressure to a constant discharge pressure, that is, in a use in which discharge pressure is constant and suction pressure varies but compression ratio is not large as in the case of a gas fuel compressor of gas turbine booster or a compressor for pressure feeding natural gas; and in the case where the screw compressor is applied in a use for pressure feeding gas to a container of large volume as in the case of pressure feeding gas to a spherical holder of city gas etc., that is, in a use in which discharge pressure varies from near inlet pressure to a predetermined discharge pressure.
2. Description of the Related Art
Variable displacement screw compressors have been used for refrigerators. In the case of a refrigerator, inlet pressure is determined according to the kind of refrigerant and the temperature at which the refrigerant is evaporated at the evaporator. That is, the inlet pressure is kept constant in accordance with the kind of use of the refrigerator but the pressure at the high pressure side of the refrigerating cycle varies according to the temperature and cooling ability of the cooling medium such as cooling water or cooling air which cools the compressed refrigerant gas to condense it at the condenser. Generally, for refrigerator a screw compressor of suitable designed-in internal volume ratio (built-in pressure ratio) is selected from among compressors of low, medium, and high built-in pressure ratio according to the conditions of operation. So, a compressor with a determined internal volume ratio must cope with a certain range of operation conditions, and the polytropic efficiency is maximum at a certain operation condition but decreases at another operation conditions.
There is a type of screw compressor of which the internal volume ratio is controlled automatically from low to high internal volume ratio in accordance with operation conditions. As such a screw compressor is generally provided with a capacity control mechanism, its construction is complicated, the control of the internal volume ratio is difficult, and high polytropic efficiency is difficult to be obtained.
In a screw compressor, the compression pressure P2 in the groove space enclosed between meshing teeth, i.e. the pressure in the groove space enclosed between meshing teeth just before it is communicated with the discharge port is related with the inlet pressure Ps and the designed-in internal volume ratio Vi as shown in the following equation:
P2=Psxc3x97Vim
where m is polytropic exponent.
When the difference between said pressure P2 and the discharge pressure Pd of the screw compressor, i.e. the pressure at high pressure side of the refrigerating cycle is large, which means excessive or deficient compression, useless work is done, which reduces the polytropic efficiency. Therefore, the designed-in internal volume ratio of the compressor is selected or adjusted or controlled so that said pressure difference is within proper value.
FIG. 8 is a diagrammatic sketch for explaining the compression process of a screw compressor of general use in a refrigerator. In the figure, as a male rotor 12 and a female rotor(not shown) meshing with the male rotor 12 rotate, gas is sucked from an inlet port 15 into the groove space formed by the meshing tooth faces of the both rotors and the inner peripheral wall of a rotor casing 14. The volume of the groove space increases as the rotors rotate, for the meshing line of the tooth faces moves toward the discharge side. When said volume becomes maximum, the communication of the groove space with the inlet port is shut, the groove space becomes enclosed, and the sucked gas is enclosed in the groove space.
As the rotors further rotate, the inlet suction side meshing line of tooth faces moves toward the discharge side to reduce the volume of the enclosed groove space to compress the gas therein. When the tooth tip 12b (in FIG. 8, only the tooth tip 12b of the male rotor is shown) reaches the beginning edge 17c of the cut-off part 17b at the discharge side end of a slide valve 17 (actually the beginning edge 17c is a beginning edge line parallel to the tooth tip 12b), the enclosed groove space communicates with a discharge port 16, and the gas in the groove space is discharged as the rotors rotate. The internal volume ratio is the ratio of the maximum enclosed groove space volume versus the volume of the enclosed space volume just before the beginning of discharge.
The capacity control for varying the flow rate of gas through the screw compressor is effected by sliding the slide valve 17 which straddles the perimeters of the male rotor 12 and the female rotor(not shown) forming a part of the internal wall surface of the rotor casing 14 and is capable of being moved in the longitudinal direction of the rotors in a way it can not be moved further to the inlet side than the slide valve stopping face 19. When the slide valve 17 is moved so that its right end 17a comes to the location shown by a chain double dashed line 17axe2x80x2, a gap develops between the right end 17a and the stopping face 19. As a result, the groove space is communicated with the inlet port 15 by way of a passage not shown communicating with the inlet port 15. The beginning of compression which is when the groove space becomes enclosed by the shutoff of communication between the groove space and the inlet port 15, becomes controlled by the right end 17axe2x80x2 of the slide valve 17.
Therefore, the farther the slide valve is moved to the left, the smaller the volume of groove space enclosed (hereafter referred to as the suction volume) and the flow rate of gas decreases.
As the beginning edge 17c of the cut-off part 17b at the discharge side end of a slide valve 17 moves to the left with the slide valve 17, the timing the enclosed groove space communicates with the discharge port is retarded and the volume of the enclosed groove space just before it communicates with the discharge port (hereafter referred to as the discharge volume) becomes smaller than when full load, i.e. when the right end 17a of the slide valve 17 is contacting with the stopping wall 19. As this decrease of the discharge volume is smaller than the decrease of the suction volume just after the slide valve 17 is moved to the left to depart from the stopping wall 19, the internal volume ratio is varied. When the slide valve 17 is moved to the left by some extent, the discharge volume which is the volume of enclosed groove space just before it begins to communicate with the axial port formed on the end face of the bearing case 14a facing the discharge side end face of rotors before the cutout part of the slide valve 17 begins to communicates with the discharge port varies with about the same rate as the suction volume, and the internal volume ratio does not vary much by controlling capacity.
Recently, as the reliability and durability of a screw compressor is superior than that of a compressor of other type, a screw compressor is required which is able to be used in the field in which a reciprocating compressor or centrifugal blower such as compressor for pressure feeding city gas to a gas turbine, compressor for boosting up the natural gas, etc. has been used.
When a compressor is used for pressure feeding city gas to a gas turbine or for boosting up the natural gas, there may be the case the discharge pressure is constant and the inlet pressure is relatively high or changes largely during operation according to use.
For example, in the case the discharge pressure is 1.8 MpaA and the inlet pressure is 0.8xcx9c1.6 MpaA, pressure ratio changes between 2.25xcx9c1.13, and assuming polytropic exponent of m=1.3, then required internal volume ratio for the best efficiency changes between 1.9xcx9c1.1. These values for internal volume ratio are largely small compared with those adopted in the case of a refrigerator. To attain pressure ratios as low as these values by a screw compressor, the designed-in volume ratio of the screw compressor must be small, that is, the dimension L in FIG. 8 must be small. But when the designed-in volume ratio of a screw compressor of variable capacity having a slide valve is too small, the suction groove space is communicated with the discharge groove space when the gas flow rate is decreased, and enclosed groove space can not be formed, leading to very low volumetric and polytropic efficiency.
The case the dimension L is small is shown in FIG. 9. In FIG. 9(a) showing the state at full load, the enclosed groove space 21 is formed as a result of the shutoff of communication of the groove space to the inlet port when the volume of the groove space is at its maximum. As the rotors rotate, the enclosed groove space 21 moves toward the discharge side while decreasing the volume, and when it reaches the beginning edge line 17c of the cut-off part 17b of the slide valve 17 and communicates with the discharge port 16, the discharging of the enclosed gas begins.
When the slide valve 17 is moved to the left to reduce the flow rate as shown in FIG. 9(b), enclosed groove space can not geometrically be formed, and the groove space 21xe2x80x2 communicates with the discharge side at the same time with the inlet side as shown by the arrow to effect no compression of gas or even if slight compression is possible the volumetric efficiency is very small.
When gas of inlet pressure of 0.8xcx9c1.6 is compressed using a screw compressor of the designed-in volume ratio Vi=2.63 for conventional refrigerator use, the pressure P2 of the enclosed groove space just before it communicates with the discharge port becomes, assuming polytropic efficiency of m=1.3, 2.8xcx9c5.6 MpaA, which is far higher than the required discharge pressure Pd of 1.8 MpaA. In this case the load by the gas pressure in the radial and axial direction of the rotors is large, and the damage of the radial and thrust bearings for supporting the load is resulted or the life of them is shortened. Also, in this case, as the difference of pressure between the discharge port and enclosed groove space just before it communicates with the discharge port is large, larger vibration and noise are resulted leading to mechanical problems. For this reason, it has been usual that the inlet pressure is lowered to that commensurate to the designed-in internal volume ratio of the screw compressor. But in this case, as the density of inlet gas is decreased, the capacity of the compressor is to be increased to secure the same flow rate as that when the inlet pressure is not lowered, leading to increased initial cost, running cost, and decreased energy efficiency.
Inventions concerning the optimization of internal volume ratio are disclosed in the past in Unexamined Published Patent Application No. 5-033789, No. 6-323269, and 2000-283071. In these inventions, as the optimization is intended with the function of controlling capacity combined, there is a limit of the optimization all over the capacity control range, and they are different from the present invention in purpose.
The present invention is made in the light of the problems cited above. The object of the invention is to provide a screw compressor capable of accommodating low compression and large pressure variation, that is, capable of being operated with high efficiency in such a condition of use.
To solve the aforementioned problems, the present invention proposes a screw compressor equipment for accommodating low pressure ratio and pressure variation characterized in that a screw compressor of variable internal volume ratio controlled by an internal volume ratio control valve is driven by a driving machine of variable rotation speed, the discharge side of the compressor is connected with the suction side of the same by the medium of a bypass control valve as needed, a computing device for calculating polytropic exponent according to the kind of gas, discharge pressure, suction pressure, discharge temperature, suction temperature, etc. is provided, and a control part for controlling the internal volume ratio control valve according to the internal volume ratio determined by the computing device.
The screw compressor is a one having an inlet and outlet port on each end side, which compresses the sucked fluid through the change of the volume formed by the meshing of a male rotor and a female rotor mounted in a casing; wherein an internal volume ratio control valve having a part for forming a part of the inner peripheral wall of the casing, straddling the both rotors and facing the outer surface of the teeth of the both rotors with a minute gap is provided movable parallel to the axes of the rotors, the control valve being movable by an extended control shaft; the suction side end of the control valve does not enter into the rotor casing side, that is, the said end is apart from or level with the suction side end of the rotors, the control valve having a cut-off part on its discharge side end part for controlling the internal volume ratio from 1.0 to a low internal volume ratio by controlling the timing of the communication of the enclosed groove space with the discharge port by moving the control valve along the axes of the rotors.
To control the capacity of a screw compressor with a reduced designed-in internal volume ratio to lower than a certain degree is, as explained before, difficult due to geometrical constraints. So, in the present invention, a slide valve for controlling capacity is not provided, instead an internal volume ratio control valve is provided, and the flow rate is controlled by controlling the rotation speed of the screw compressor. In the case the discharge pressure is constant with varying suction pressure or in the case the suction pressure is constant with varying discharge pressure, in which accordingly the compression ratio varies, the internal volume ratio is controlled so that the polytropic efficiency is maximum by moving the internal volume ratio control valve according to the value determined by the computing device which calculates the polytropic exponent in accordance with the kind of gas, discharge and suction pressure and temperature, etc.
Further, by starting the compressor with the internal volume ratio adjusted to low values near 1.0, the starting torque is reduced to evade a state of impossibility of starting and the load to the driving motor and bearings are alleviated.
In the case radial bearings of the rotors of a screw compressor is sleeve bearings, it is preferable not to operate continuously under low rotation speed below a certain speed because long-operation under low rotation speed induces the wear and burn-out of bearings as the generation of oil film is difficult due to the low peripheral speed of bearings. Instead, it is preferable to reduce the discharge gas flow rate by controlling the flow rate of the bypass gas from the discharge side to the suction side by the bypass control valve provided on the passage connecting the discharge side with the suction side.
In the case the suction and discharge pressure is constant, as the compression ratio is constant, it is suitable to apply a compressor of fixed internal volume ratio with which the polytropic efficiency is maximum at the said compression ratio or a compressor of which the internal volume ratio is adjusted to give maximum polytropic efficiency at the said compression ratio. When the fixed internal volume ratio compressor is applied, a screw compressor is used of which the fixed internal volume ratio is the internal volume ratio in full load in the condition of the use.
Although an engine (with a clutch and controlled stepwise by change gear) or others can be used for the driving machine, an inverter motor of which the rotation speed is controlled by varying the frequency is suitable, for the stepless control of the gas flow rate is easy.